The present invention broadly relates to the fabrication of covering material, e.g. for floorings or floor coverings and, more specifically, pertains to a new and improved method of fabricating highly compressed covering material by pressing a lumpy plastics material or a plastics pre-product during heat action. The present invention also relates to a new and improved apparatus for performing the inventive method.
Highly compressed floor coverings made of a plastics material were the response or rather the reaction to the ladies' high-heeled shoes that came into vogue in the early fifties and continued in fashion the greater part of the sixties. Such highly compressed flat or planar products possess a substantially homogeneous structure and have mechanical properties capable of meeting heavy-duty strain and very rough handling. Particularly, a high abrasion resistance as well as a high resilience are requisite for a resilient flooring. Such highly compressed material can be also fabricated in a conductive manner or form which is advantageous for various uses or applications.
Highly compressed floor coverings are hitherto fabricated according to discontinuous batch-processing methods and in presses operating in batch quantities. The lumpy plastics material, which is used as the starting or basic material and provided, for example, in the form of pellets or chips or bits, is brought or fed into a container. A cover is placed on this container, whereupon the container and the cover are put in place between the plates of a press. Simultaneously with the pressing operation, the container is also heated. The process is always carried out with an excess of material which, upon reaching the given or predetermined material volume between the container rim and the cover, can escape or exit from the container. In order to minimize loss or waste of material and avoid rejects or scrap, the starting material must be accurately weighed-in prior to each pressing or molding operation or cycle. According to this known method, the process is performed with pressures between 30 bar and maximum 50 bar. The resulting blocks having, for example, a thickness of 14 mm, are then split into thinner plates or layers. The individual plates are ground and tempered, and cut to smaller sizes and ultimately installed or laid. The produced plates or layers are substantially non-directional, i.e. they possess the same properties in all directions. Such thin plates are suitable as a floor covering, but also as a covering or cover material for other applications or purposes, where they are subjected to heavy-duty strain or load, for instance the loading area of fork lift trucks. Such covering material is so durable that the connection or joining element between the fork lift truck and the plastics covering is sooner damaged than the plastics covering itself.
This prior art method for discontinuous batch processing is disadvantageous in that, apart from the weighing in of each individual batch, the transfer or conveyance of the material, which is to be pressed, to the press uprights, which are arranged in tandem and independent of each other, represents a relatively time-consuming and manually executed preparatory operation. Accordingly, the discontinuous process is uneconomical in every respect. Furthermore, only covering plates of a certain size and of an average thickness can be fabricated. There is also the possibility of rejects when the transfer of the material to be pressed from one press upright to another is not effected rapidly enough. In other words, the quality of production is strongly dependent on the aptitude and the alertness of the operating personnel.
According to a continuous method known to the art, the floor coverings are fabricated from calandered foils by lamination on a steel roll. The required calandered foils must be already available in a highly compressed form. This means that the lamination process is preceded by a relatively energy-consuming pressing or compressing step. As a result of calandering, the properties of the laminated products in the longitudinal direction differ from those in the transverse direction. The mechanical properties thereof are substantially inferior to those of homogeneously pressed coverings. The thickness of these floor coverings is limited, on the one hand, because generally only two to three foils of approximately up to 1 mm thickness can be used and, on the other hand, because--according to belt or roll material--work on a laminating machine or laminator can be carried out only in the pressure range of 2 to 6 bar. In the case of several foils, the heat required to bond the individual foils is substantially weaker towards the center of the floor covering as a result of self-insulation. Accordingly, the melt connection or fusion between the individual layers possesses only a limited wear resistance. Furthermore, since the melt-on of the material in the fringe regions of the layers takes time, this prior art continuous process is relatively slow and thus time-consuming.